This invention relates to solid state gas sensors. More particularly, this invention relates to solid state gas sensors for sulfur dioxide. Even more particularly, this invention relates to solid state sulfur dioxide sensors using Al—Al2O3—Au structures.
Sulfur dioxide (SO2)is a gas that is both useful in industrial applications and an undesired byproduct of some processes. For example, SO2 is used to produce cooking liquors for paper making, but it is also considered a pollutant from lime kilns. Government mandates limit the amount of SO2 that may be emitted from the paper making process.
SO2 is also a useful compound in the wine making industry, where it is used to delay bacterial growth. However, it also is a byproduct of yeast fermentation and SO2 levels in wine can vary with temperature and pH. This variation may adversely affect the quality of the final product. Therefore, effective monitoring and control of SO2 levels is generally recognized as essential to all phases of wine making. The standard methods of monitoring SO2 have been the Ripper or iodine method and the vacuum aspiration method. Both of these methods are unsuitable for testing on location, requiring that samples be taken from a cellar to a laboratory for analysis. This may cause a significant delay before corrective dosing, if necessary, can be effected.
The Ripper method is also susceptible to several sources of error. Phenolic substances in red wines, for example, react with the reagent iodine to produce results that indicate a higher level of SO2 than is actually present. The end point of this test is also not well defined and the results tend to fade quickly. The Ripper method is also susceptible to skewing by ascorbic acid. Additionally, juice from grapes affected by botrytis cannot be accurately measured by iodine titration. Furthermore the iodine reagent is unstable and must be standardized by titration with sodium thiosulphate periodically. Iodine reagent is also extremely sensitive to sunlight.
SO2 plays a role in many other industries as well. For example, the ability to minimize emissions of SO2 from aircraft may also have an impact on the ability of the aerospace industry to develop new supersonic transport vehicles. Furthermore, SO2 is corrosive of some combustion engine components. SO2 has also been shown to play a role in fouling catalysts used in the automotive and petroleum industries. SO2 is also generated during the regeneration of sorbents for coal gasification. Therefore, there is a need to monitor SO2 levels in a wide variety of industries, where a lack of appropriate chemical sensors can be a limiting factor for many technologies. This is especially true in the case of sulfur dioxide (SO2) monitoring.
Recent efforts in the area of gas detection incorporate solid electrolytes, metal oxides, or polymer coatings as the detectors' active region. Gas detectors utilizing solid electrolytes are disclosed in a number of U.S. patents. For example, U.S. Pat. No. 4,855,034 discloses a sulfur dioxide sensor which utilizes a solid electrolyte of a compound of sodium oxide and aluminum oxide (β-alumina). The sensor also includes a platinum, lead, or platinum-lead alloy which accelerates the reaction of sulfur dioxide with oxygen.
U.S. Pat. No. 6,179,992 discloses a gas detection systems that contains an oxygen ion conducting solid electrolyte and a metallic salt which acts as a gas sensitive layer. A cationically conductive material is disposed between the electrolyte and the gas sensitive material. U.S. Pat. No. 6,200,445 also discloses a sulfur dioxide sensor that comprises a solid electrolyte that has oxygen ion conductivity. A detecting electrode is electrically connected to at least part of the surface of the solid electrolyte, and a basic electrode is also connected to at least a part of the surface of the solid electrolyte. The detecting electrode contains glass and either gold or a gold alloy. The basic electrode contains platinum or a platinum alloy. The glass component of the detecting electrode is reported to suppress reaction of inflammable gases such as carbon monoxide. A similar sensor is also disclosed in U.S. Pat. No. 6,368,479.
U.S. Pat. No. 4,718,991 discloses a “proton conductor gas sensor” for detecting gases, such as sulfur dioxide, which produce protons upon reacting with water. The gas sensor comprises a proton conductor which may be antimonic acid, zirconium phosphate, dodecamolybdophosphoric acid and the like. Attached to the proton conductor is an ionization electrode and a reference electrode. The ionization and reference electrodes may be platinum, rhodium or other metals. Silver and gold are also listed as potential materials for the reference electrode.
The use of metal oxides in gas sensors is also known. For example, the use of a zirconium oxide probe to measure sulfur dioxide levels in a combustion system is disclosed in U.S. Pat. No. 4,978,434.
A system utilizing thin film electrodes coated with an electrolyte film is disclosed in U.S. Pat. No. 5,716,506. The thin film electrodes may be platinum and the electrolyte film is capable of conducting electricity at room temperature. The sensor comprises a substrate which may be silicon dioxide, alumina, or a polymer, a working electrode deposited on the substrate, a counter electrode also deposited on the substrate and a film of polymer electrolyte applied over both electrodes. The working electrode comprises a first layer of gold, platinum or carbon which is in contact with the substrate and a second layer of platinum in contact with the first layer. The first layer has a thickness of about 250 to about 5000 angstroms.
Other types of sensors for the selective detection of gases are also known. U.S. Pat. No. 5,841,021, discloses an electrochemical gas sensor for detecting a variety of gases including oxides of sulfur. The sensor has an electrode which reacts to the presence of the gas in question, a reference electrode which does not react to the gas in question, and an electrically conducting substrate which connects the two electrodes. A gas sensor is disclosed in U.S. Pat. No. 6,165,336 which utilizes a gas permeation element which allows the separation of a gas of interest such as carbon monoxide from gases that may cause deterioration of the sensor. U.S. Pat. No. 5,041,204 is directed to an electrochemical method for detecting sulfur dioxide or hydrogen cyanide using copper ions. U.S. Pat. No. 5,128,018 is also an electrochemical apparatus for detecting gases such as sulfur dioxide. This system makes use of heteropoly acids or iron salts in an electrolyte in an electrochemical measuring cell. U.S. Pat. No. 5,041,204 discloses a electrochemical measuring cell for detecting hydrogen cyanide or sulfur dioxide using a pair of electrodes disposed in an electrolyte.
The adsorption of SO2 onto clean metal surfaces is known. For example, it is known that room temperature adsorption of SO2 on copper surfaces is dissociative, forming adsorbed S(a), O(a), and SO(a) species. However, a method for using gas adsorption onto metal surfaces in connection with tunnel junction geometry for devices has not been known.
Therefore, there is a continuing need for alternate methods of detecting gases such as SO2. There is also a need for a gas sensor, especially a sensor for SO2, that is portable and easy to use. There is a continuing need for detectors that are smaller, lighter in weight, and require less power than present day detection schemes. There is a particular need for SO2 detectors in the wine industry that provide results with a minimum of delay from the time of taking a sample, and that are easy to use.